DEVELOPMENT OF MYXOBOLUS-NEUROPHILUS (GUILFORD, 1963) (MYXOSPOREA) IN THE BRAIN OF YELLOW PERCH (PERCA-FLAVESCENS) IN VINEGAR LAKE, NOVA-SCOTIA

The development of Myxobolus neurophilus (Guilford, 1963) in the optic tectum of yellow perch, Perca flavescens, inhabiting Vinegar Lake, Nova Scotia, is described. Young perch acquired infections during July when 2-3 months old. By early August, small plasmodia occurred in the cortex and consisted of an outer thin ectoplasm and inner endoplasm containing sporoblasts. By the end of August, plasmodia were found in the brain cortex and periventricular layers. They measured up to 310 mu m in diameter and contained sporoblasts and fully developed spores. Some plasmodia had ruptured and released spores within the periventricular layer. It is estimated that in young-of-the-year fish the development of M. neurophilus requires 40-50 days. Prevalence was 100% in fish older than 1 year, with infections consisting of spore masses located in the periventricular layer. Spores were either free in tissue spaces or within unidentified mononuclear phagocytes. Spores were absent from other organs, and spores are probably released when the host dies or when it is eaten by a predator. Macroinvertebrates did not play a role in spore release from carcasses.